Misdirected energy: Hong Kong City University roof collapse highlights danger of adding vegetation in a bid to go green

Adding greenery may be too much for a roof to bear, compounded by laxity over the submission of building plans for structural changes

The giant rooftop that collapsed at a City University sports centre and left three injured has highlighted the potential threat of adding rooftop vegetation, a novel way to fight the heat-island effect, to old buildings.

The accident, which could have injured hundreds of people originally scheduled to attend a dinner event on Saturday night, also called into question the lack of government supervision of this kind of rooftop vegetation, which is promoted by the Environment Bureau.

At the City University, the vegetation was understood to have been added last year to the top of Chan Tai Ho Multipurpose Hall, which was completed back in the 1990s.

While the roof was not designed to hold anything substantial – as indicated in the building plan submitted to the government in 1989 – vegetation that would have required a roof five times stronger was nonetheless planted last year, as part of the university’s pledge to go green.

“The figures showed that the rooftop was not supposed to hold a lot of [vegetation],” said Vincent Ho Kui-yip of the Institute of Surveyors.

Ho said the current building regulations relied heavily on owners’ own initiative in submitting a plan for approval if they altered a building’s structure. But the Buildings Department would never know if owners skipped this procedure.

He said the department should remind owners to resubmit plans for new structures.

Professor Jim Chi-yung, an expert on urban soil science and a staunch advocate of green roofs, said it would be “very risky” to install a green roof on a structure – especially an existing one – that did not meet loading capacity standards.

“The roof must be able to take the weight of the dead loads of soil, vegetation and drainage as well as the life loads, which include people walking on it,” Jim said. “The loadbearing capacity must be bigger than the sum of the dead load and life loads.”

He added that the drainage design for a green roof could be an Achilles heel, as it was often not done properly. Poor drainage could lead to water gathering on the roof, leading to dangerously high loading which could jeopardise the roof structure.

But experts asked the public not to panic over the environmentally friendly measure.

“It is already an accepted practice around the world,” said Leung Man-kit of the Green Building Council’s policy and research committee.

City University said on its website that the green roof top “could achieve an energy saving of about 60 kWh/sq m per year, a reduction in CO2 emissions of about 3.2 tonnes per year, [equivalent] to planting 137 trees”.

The Buildings Department could not confirm whether a new plan was submitted before the university added the rooftop vegetation. The university said on Friday night that the contractor had made proper assessments.